(1) Field of the Invention
The present invention relates to an image guide having a multiplicity of optical silica glass fibers fusion bonded together in side-by-side relation.
(2) Description of the Prior Art
Images guides which comprise a multiplicity of optical fibers formed into a bundle-like assembly are used for transmitting image information from one end thereof to the other.
Known approaches in practice to permit transmission of a clear image by means of an image guide are that the diameter of each of the individual optical fibers constituting the picture elements of the image guide is made as small as possible, and that the number of picture elements in a unit sectional area is made as great as possible. It is also a known practice that in order to reduce possible image distortion, the optical fibers are closely arranged and assembled in such a way that no gap is present among any adjacent fibers and that the individual fibers have their both ends positioned in exact opposed relation.
To give an example of such image guide already in practical use, there is an image guide made of multi-component glass, which is employed in an optic scope such as photogastroscopic applications. Multi-component glass can be easily formed into many optical fibers, each having a uniform outside diameter because it has a low melting point, and therefore the fibers are substantially uniform in diameter. So an image guide of the multi-component glass type can be produced in such a way that the optical fibers are densely bunched together simply by being put in alignment. However, it is impracticable to produce a long image guide of such type because of its high transmission loss. Moreover, the multi-component glass type is not heat resistant, nor is it radiation resistant, which fact renders it unsuitable for observation use in such an abnormal atmosphere as in a blast furnace or in a nuclear power plant.
In contrast with such type using multi-component glass fibers, an image guide using optical silica glass fibers which involve a lower degree of transmission loss and has good heat resistance and good radiation resistance permits observation in above said abnormal atmosphere; and moreover, optical silica glass fibers enable to produce a long image guide.
Now, optical silica glass fibers cannot be produced by drawing according to a double crucible method which is adopted for drawing of multi-component glass fibers, because silica glass has a high melting point. This makes it difficult to obtain fibers of uniform diameter. If a multiplicity of silica glass fibers are assembled into a bundle form in fabricating an image guide, therefore, it is difficult to obtain a bundle in which the multiplicity of fibers are densely arranged so that no gap is present among adjacent fibers, simply by putting them in alignment, because of the lack of diametral uniformity among the fibers. The presence of such gaps is undesirable because it lowers the quality of the transmitted picture.
So, as a method of fabricating a silica glass image guide which has a larger number of picture elements in a unit sectional area, it has been proposed to bunch a multiplicity of optical fibers together, subject them to heating and drawing so that each of the individual fibers is fusion bonded to ones adjacent thereto, thus arranging the multiplicity of fibers into an assembly so that no gap is present among any adjacent fibers.
An image guide produced by this method is such that each of the optical fibers is fusion bonded to ones adjacent thereto; and in such image guide in which the optical fibers are highly densely arranged, the individual optical fibers are extremely fine, and this, coupled with the fact that they are fusion bonded together, each to ones adjacent thereto, substantially reduces the light shielding effect of the cladding, or the portion having the role of cladding, of each optical fiber. Therefore, when such image guide having an assembly of optical fibers fusion bonded together is used, there is a problem that the ingress of light beams from external sources into the core of each optical fiber reduces the contrast of a transmitted picture, making it difficult to obtain transmission of a picture of good visibility.
As an approach to solve this problem, providing about each optical fiber a thin layer or coating of light absorbing glass to prevent light transmission between adjacent cores, as disclosed in U.S. Pat. No. 2,825,260, may be thought of, but provision of such light absorbing layer or coating on an optical silica glass fiber is impractical since a light absorbing glass having a thermal coefficient of expansion comparable to that of silica glass, a material having a very small thermal coefficient of expansion, is not available and since such coating, if provided, may easily come off.
In order to solve said problem, an attempt has been made to use an image guide as inserted in a pipe of a light sealing material, e.g. a stainless steel pipe, but one difficulty with this approach is that an air layer present between the inner surface of the pipe and the surface of the image guide may irregularly reflect optical cross talk from each optical fiber for the mirror effect thereof, and the irregularly reflected light in turn may re-enter the cores of optical fibers, so that the visibility of transmitted pictures may be adversely affected. Another difficulty is that such air layer is likely to act as a medium for transmission of light entering through the inlet of the stainless pipe, thus degrading the contrast of the transmitted picture. Furthermore, light incident upon the core at a larger angle of incidence than a numerical aperture determined by a reflective index differential between the core and the cladding of the optical fiber breaks through the core into the cladding, then reaching the outer periphery of the image guide, after reflected by the outer periphery, the light goes back into the image guide. Through such process of repeated reflection between one side and the other of the outer periphery, said light is transmitted through the image guide.
Such incident light degrades the quality of the transmitted picture. Moreover, in the course of such repeated reflection on the outer periphery, the angle of incidence of such light relative to the optical fiber may change to cause such light to transmit noise through the core. Thus, a faithful transmission of a picture cannot be achieved.
As above stated, an image guide consisting of optical silica glass fibers, each fusion bonded to others adjacent thereto, has advantageous features such as being less liable to transmission loss, highly heat resistant and highly radiation resistant, on one hand, but on the other hand, it involves factors which may adversely affect the quality of transmitted pictures, and this has prevented the practical application of such image guide.